Selective laser sintering (“SLS”) is an additive manufacturing technique that uses electromagnetic radiation, for example from a laser, to fuse small particles of plastic, metal (direct metal laser sintering), ceramic, or glass powders into a mass having a desired three dimensional shape. The laser selectively fuses powdered material by scanning cross-sections generated from a three dimensional digital description of the part on the surface of bed having a layer of the powdered material disposed thereon. After a cross-section is scanned, the bed is lowered by one layer thickness, a new layer of powdered material is disposed on the bed, and the bed is rescanned by the laser. This process is repeated until the build is completed.
Prior to scanning, an SLS machine typically preheats the powder material disposed on the bed to a temperature proximate to a melting point of the powder. Preheating is typically accomplished by heating the actual bed, which transfers energy to the powder in the form of heat via thermal conduction. Preheating the powder makes it easier for the laser to raise the temperature of powder to a fusing point.
When working with certain material in the SLS process, for example polymer powders, the bed temperature is set to a temperature specific to the polymer resin in use. This specified temperature is typically proximate to the melting point of the polymer resin. The laser causes fusion of the powder in locations specified by the build input. Laser energy exposure is typically selected based on the polymer in use and is between the amount required to fuse the resin and the amount that will cause degradation. Preheating of the material inhibits unwanted distortions in formed parts during cooling.
After the layer-wise process is completed, the formed object(s) is disposed in a volume of unfused powder, referred to as a cake. The formed object(s) is extracted from the cake. The powder from the cake that is not fused into the built part can be recovered, sieved, and used in a subsequent SLS build.
Polyaryletherketones (“PAEK”) are of interest in the SLS process because parts that have been sintered from PAEK powder are characterized by a low flammability, a good biocompatibility, and a high resistance against hydrolysis and radiation. The thermal resistance at elevated temperatures as well as the chemical resistance distinguishes PAEK powders from ordinary plastic powders. A PAEK polymer powder may be a powder from the group of polyetheretherketone (“PEEK”), polyetherketone ketone (“PEKK”), polyetherketone (“PEK”), polyetheretherketoneketone (“PEEKK”), or polyetherketoneetherketoneketone (“PEKEKK”).
The bed temperature setpoint may be determined, for example, by referring to a temperature setpoint published by a vendor of the powder. In such circumstances, the operator sets the SLS bed temperature to the setpoint specified by the vendor and commences the SLS build process when the bed has achieved the setpoint temperature.
A disadvantage of relying on a bed temperature setpoint specified by a vendor is that the melting point of the powder may vary between different lots of powder. This is true even if between different lots of the same type of powder. As a result, the temperature setpoint specified by the vendor may be incorrect for the actual lot of powder being used in the build.
Another disadvantage of relying on a bed temperature setpoint specified by a vendor is that the vendor provides a setpoint for a lot of pure powder. Typically, a vendor does not provide a bed temperature setpoint for a lot subsequently prepared by the operator by, for example, combining two or more types of powder, for example two different polymers. Similarly, a vendor typically does not provide a bed temperature setpoint for a powder having one or more fillers.
Another disadvantage of relying on a bed temperature setpoint specified by a vendor is that a melting point of unused powders versus a melting point for recycled powders can vary dramatically. As a result, it is necessary to use different bed temperature setpoints depending on whether an SLS powder lot consists of unused powder, first recycle powder, second recycle powder, or some combination thereof. For example, as disclosed in U.S. application Ser. No. 13/705,332 to DeFelice et al., the difference between the bed temperature setpoint for unused PEKK powder and first recycle PEKK powder is fifteen degrees Celsius. The '332 application to DeFelice is hereby incorporated by reference.
Another disadvantage of relying on a bed temperature setpoint specified by a vendor is that certain polymer powders, for example PEKK, are copolymers. A copolymer comprises two (or more) monomeric species. For example, PEKK is a copolymer (AB type EKK/EKK). In lots of such copolymers, the ratio of a first species compared to a second species may be varied to achieve, for example, blends having different ratios. A problem with such copolymers is that the temperature at which fusing initiates may vary based on the ratio of the first species relative to the second species, thus making it difficult to select a correct bed temperature setpoint.
As a result of these disadvantages associated with bed temperature setpoints specified by a vendor, incorrect bed temperature setpoints are often used in SLS runs. An incorrect bed temperature may results in serious structural problems in the part formed during the SLS procedure. For example, if the bed temperature setpoint is too low, the built part may become distorted relative to the desired three-dimensional shape. If this happens, the built part may be discarded, or it may require additional man hours to further shape the part so that it conforms to the desired three-dimensional shape, to the extent reshaping is feasible.
If, on the other hand, the bed temperature setpoint is too high, the powder may began to melt or fuse prior to being sintered in the layer wise fashion. This can result in a built part with substantial structural flaws, and, if the bed temperature setpoint is above a certain temperature setpoint it may prevent formation of a three-dimensional part from during the SLS process because successive layers will not fuse together.
It is known to overcome the problems associated with reliance on a bed temperature setpoint specified by a vendor by observing properties of a powder in an SLS machine during a warmup cycle. In this observation method, a layer of powder is disposed on a bed at a temperature well below the melting point of the disposed powder, for example room temperature. The bed temperature is then increased and an operator visually observes the powder on the bed for certain visual cues that indicate the onset of fusion, or that indicate that fusion is imminent. For example the color of the powder and the texture of the powder may shift, indicating that the layer of powder is beginning to fuse. When these visual cues are observed, the operator notes the temperature of the bed. The bed temperature setpoint for the lot being visually tested is typically between five to eleven degrees Celsius below the temperature at which the layer of powder begins to fuse.
A disadvantage of the above described observation method is that it relies of the visual acuity of the operator conducting the calibration. As a result, and due to differences between different operators, an operator may select a bed temperature set point that is too high or too low.
Another disadvantage of this method is that certain environmental factors, for example the type and intensity of lighting proximate to the bed may also affect the visual observations made by the operator. It has been found that the structural properties for sintered powders can be statistically affected by shifts of as little as a single degree Celsius.
It is an object of the present invention to overcome these disadvantages and other disadvantages associated with the prior art.